Removing the mags, once the engine's internals are correctly positioned,
is really just a simple disassem-bly job. It's getting
the mags back onto your engine and properly timed that's the interesting
part. But that's not a complicated process, either. In
fact, it will only take about an hour for removal and about an hour and
a half for re-installation.
Only two special tools are needed for the job. The first is Slicks
Timing pin, #T-118. You can get one from any shop that
overhauls Slick Mags. Or call Slick at 815-965-4700. They'll drop one in
an envelope for you free of charge. Your A&P
might even have a extra one kicking around in his toolbox. If you're
getting a new mag, Slick always packs a timing pin along
with it. The T-118 is used to set the mag's distributor gear position so
it's ready to be installed on the engine.
The second special tool is a magneto timing box to time the mags to your
engine. You can buy one from a number of tool
suppliers in Trade-A-Plane for about $40.00. Or borrow one from a
generous A&P you know. Either way, the other tools
are just the standard ones.
Getting Prepped
Whether the engine's a Lycoming or Continental, the process is basically
the same. Check that the magnetos are both
switched off and the mixture and throttle controls are at lean cutoff
and idle positions, respectively. Then remove the top
cowl. Visually check that both P leads are, in fact, still connected to
the magnetos. The P-lead and the ground wire are the
only wires connected to the mag other than the thick ignition wires, if
the mags are the impulse coupling type. If the mags are
the retard breaker type, there's another wire that connects the breakers
to the starting circuit. Each mag should have all of
these wires connected. Even then, to be on the safe side always treat
the prop as if the engine could still start as soon as the
prop is budged. To some, this may seem a bit overcautious. But because a
mag is connected on the outside, doesn't mean it's
connected on the inside. Besides, it's just as easy to move the prop
while staying out of it's arc of travel, as it is to be in the
arc and in potential danger.
An engine is timed at a specific number of degrees BTC (Before Top
Center). It fires the spark plugs for #1 cylinder at this
point. Once this is set, the timing on all the other cylinders follows
suit. So finding out what the timing number is for your
engine is the next step. This information is in the engine's manual and
stamped on its data plate. The data plato on Lycomings
is located on the lower right hand side of the engine. But unless your
really good with a mirror, it's necessary to remove the
bottom cowl to read it. Continentals are easier because they have their
data plate on the upper right side of the engine. For
our O-320, the timing mark is at 25 degrees BTC. It's usually a number
between 20 and 25.
Next locate the two timing marks. On Lycomings, one or the marks is on
the outer edge of the starter flywheel, which is
mounted to the prop. Look for a line on the outer edge of the flywheel
with the timing number stamped next to it. The other
mark is the top center seam of the two engine case halves.
Continentals are not that simple. The timing marks could be in a number
of places or not be there at all. I guess it's the price
you pay for having an easy to read data plate. Many Continentals have a
bronze, safety wired, one-inch plug that's right
behind the prop, on the left side of the engine case. A timing mark is
stamped on the gear which is easily visible after the
plug is removed. Rotate the engine slowly to give the oil a chance to
run off the gear and reveal the marks. Don't be
concerned if the number you see is 23 and there's a line marked 25 and
another marked 20. Just extrapolate where 23 will be
between them and use that. The other mark is the center line of the
hole.
If your not lucky enough to have a Continental with a timing plug, it's
time to start hunting, On Some models, the timing mark
is on a pulley on the accessory case at the rear of the engine. Other
models have different locations. Still other models have
no mark at all. A degree wheel must be mounted on the prop spinner and
synchronized with the No. 1 piston's Top Dead
Center (TDC) position. The BTC is figured from there to get a timing
mark. You must check the maintenance manual for your
particular model engine to find out where your marks are, if you don't
already know. Or maybe ask your A&Phe'll probably
know just by looking at it. If he starts laughing when you ask him,
you've probably got the one without marks.
Setting Up The Engine
Once the timing marks are located, the next step is to line them up. Do
this by first finding the No. 1 cylinder. On a Lycoming,
No. 1 is the front cylinder on the right side of the engine, viewed
facing the engine from the front of the plane (i.e., looking
back towards the tail). For Continentals, No. 1 is always the right rear
cylinder. Or just look an the engine. Both makes have
every cylinder's number right on the engine case.
Once you locate No.1, remove the top spark plug and lay it on top of the
cylinder. Then put your finger over the plug hole
and rotate the prop in the direction of normal rotation. Remember to
stay out of the prop's arc of travel. As the No. 1 piston
comes up on its compression stroke, it will force air out of the open
plug hole. Listen and feel for this air pressure. When it
starts to bleed by your finger, forget the piston and watch for the
timing mark to come into view. As soon as you see it , stop.
Move the prop the rest of the way into alignment by lightly tapping the
trailing edge of the prop blade with the palm of your
hand. In this way, bring the moving timing mark to align exactly with
the fixed timing mark. Do this slowly. If you go past the
fixed mark, you can't just tap the prop back a little. You will lose
proper internal alignment due to gear lash. You must come
up on the mark when moving in direction of rotation. If you do go past
it, move the prop blade back about an eighth of a turn
and come up on the mark again. Also, if you have an impulse-coupling
type mag (usually there's only one), the prop must first
be moved forward, past the fixed mark, until you hear the impulse
coupling pop. Then move the prop back an eighth of a turn
and come up onto the mark. Be careful not to go too far back or you'll
engage the coupling again. If you don'' know if you
have an impulse-coupled mag or not, just go past the mark and listen for
the pop or snap sound as the coupling slips and
spins the mag. No pop means no impulse coupling (or a defective one, in
which case you'll probably removing the mag
because the engine won't start).
Mag Removal
Once the timing marks are perfectly aligned, the mags can be removed.
You may want to put a sign on the prop blade
warning people not to move it, especially if the plane's going to be
left unattended for a while.
Then, pick a mag and remove the three screws that hold the ignition
harness cap on. Put an alignment mark on the cap to
show which end is up and make it easier to correctly position it during
reassembly. The cap must go back onto the mag
properly orientated. Once free, move the cap out of the way as much as
possible. Then disconnect the P-lead and the other
wire or wires from the mag. It's true that once you remove the P-lead,
the mag is on or "hot." It's still safe, though, because
the ignition harness cap has been removed. (Besides, what are you doing
moving the prop, anyway?)
Note how the magneto is positioned on the engine. It will go back in at
about the same angle, in relation to the surrounding
parts. On the Cessna 172 with the O-320 E2D, for example, U use the
vacuum pump as my reference point. It's conveniently
located right between both magnetos. Some mechanics like to scribe a
small line from the mag mounting flange to the
accessory case as a reference mark.
Remove the two nuts, lock washers and clamps located at the base of the
mag that secure it to the engine. A ?" open end
wrench will do the job (a long, locking extension and a universal socket
make this a whole lot easier). Usually, the nuts will
not have enough room to come off the engine stud unless you move the mag
off the engine case a little. If the mag has an
impulse coupling, there will be about a one-inch thick base plate
between the mag and the engine case. Be sure not to disturb
this plate. If it breaks loose from the engine case, even a little,
remove the plate, once the mag is off, and reinstall it with a
new gasket. (Any motion of the impulse base plate usually damages the
gasket, which means you'll be back doing this all
again just to change the gasket. Deal with it now if you have to.)
Once the mag's hardware is removed, carefully remove the mag from the
engine. Handle it gently. There are two rubber
bushings that are on the shaft end, inside the engine. This set of
bushings rests in a oval-shaped steel retainer. If handled
roughly, they will fall out of the retainer and into your accessory
case, completely ruining your day. Being rubber, they can't
be retrieved with a magnet, thus they are not easy to remove from the
case. Other than disassembling the engine, the only
other way is to fish around the case for hours trying to catch them with
a grabbing tool of some sort.
Once you've carefully removed the magneto, look into the accessory case.
Hopefully, and usually, the bushings are still in the
retainer. Carefully remove the bushings and lay them safely aside. Then,
mark each mag left or right so they are not swapped
when reinstalled. If new ones are to be installed, look at the data
plate that's riveted to the mag's side. Get the model and
serial number. Also, find the box market "ROT" for rotation. It will
have a "R" or a "L" stamped in it. Note this info to know
which mag goes into which access hole during re-installation.
Clean all the old gasket from the engine case. You want a clean, dry
gasket surface for the new gasket. Don't let anything fall
into the case opening. Stuffing a clean rag into the hole will help.
Remove the other magneto in the same way. If the lock
washers are the split ring type, they can be used again. Otherwise, get
new ones. Always use a new gasket.
Setting Up The Mags
When the magnetos are ready to go back onto the engine, they have to be
positioned to fire on No. 1 cylinder. So, with mag in
hand, look at the mag's data plate and find its rotation direction.
Armed with this vital info, go to the top of the mag where the wiring
harness attaches. Looking down on the top of Slick
mags, you will see two holes in the distributor block. One is marked
"L", the other, "R". On a mag that has a "L" stamped on
it, insert the No. T-118 timing pin into the hole marked "L". (If the
people at Slick make this any easier, I'll be out of a job.)
Then gently and slowly rotate the drive gear, while trying to gently
slip the timing pin deeper into the hole. When the pin
fully seats in its timing hole, the mag will no longer turn. Don't force
it. The distributor gear is now correctly positioned and
ready to be installed.
Leave the timing pin in the mag until it's mounted on the engine. The
pin will prevent it from turning during installation. But
be careful not to turn or put pressure on the drive gear with the timing
pin installed. It will screw up parts in the mag. If
you've got two mags going back on, set up the other one after this one
is mounted.
Installing The Mags
Before installing the mags, check to see that the engine's timing marks
are still lined up. Some bonehead might have turned
the prop while you were gone. If it has been moved, remove the rags and
reposition the timing marks as you did before. It
doesn't matter that the mags are out. If all's well, remove one rag from
the access hole.
Then reinstall the two rubber bushings into their retainer. You may want
to give them some incentive to stay in place by
putting some grease in the retainer and on the bushings. Note that the
bushings have a squared edge and a rounded edge the
rounded edge faces out.
Be sure you have the correct mag for the access hole you're working on.
Next, look a the angle of the retainer in the engine.
The magneto must be orientated the same way for the mag'' drive gear to
slip between the bushings. Don'' try to move the
mag's' drive gear for alignment. The timing pin will prevent it from
turning. Instead, rotate the entire magneto in your hand so
it's properly oriented. Remember how the mag was positioned in regards
to the vacuum pump or whatever reference point
you used. That should give you the proper orientation.
Slip a new gasket onto the mag and then carefully install the mag into
the engine case. Don't force it in. When the mag's drive
gear is properly lined up with the bushings, it will just slip into
place. The drive gear and bushings are aligned and fully
engaged when the mag is fully seated against the engine case. Slip on
the hold-down clamps, lock washers and nuts.
Remember, to get the nuts on the mag, it may need to be moved back off
the engine case a little. Don't go too far or the drive
gear will be disengaged from the retainer and bushings. Thread the nuts
down, hand tight. Then remove the timing pin.
Tighten the nuts so the mag can be rotated about its base, but is still
held snug against the engine case. Set up the other mag
with the T-118 and install it in the same way. Don't worry about fine
tuning the timing yet. Get both mags in first. Don't hook
up any wires or harnesses yet, either.
Fine Tuning The Timing
Once both mags are mounted, get the timing box. Hook up the box's black
wire to ground, which is anything metal on the
engine. The other wires go to the mag's P-lead terminal, one wire per
mag. Then switch on the box and slowly rotate the
mag. The timing box will indicate that the mag's points have just opened
and the mag is in its properly timed position by the
light going on or off, and/or an audible buzzer sounding, depending on
your model timing box. Do the same to the other mag.
The next step is to check the timing job. Do this by moving the prop in
reverse rotation for about an eighth of a turn. Then, by
tapping the prop with your hand, as you did earlier, move the prop until
the timing box again indicates the points have
opened. Now check the engine's timing marks. Are they aligned?
Usually one magneto goes off before the other one by a fraction of a
rotation of the prop, so don't be concerned. If the marks
are aligned, continue tapping the prop until the other mag goes off and
check the marks again. It is very normal for the mags
to go off at slightly different times and have the timing marks aligned.
If a magneto went off too early or late, rotate the mag
slightly. Don't be concerned that you may rotate the mag in the wrong
direction. Then do the timing check again. Is it better or
worse? Turn the offending mag accordingly. Do this until the mags are
properly timed to the engine. How far you want to go
with this is up to you. If you want both of them to go off at exactly
the same time, so be it.
When the mags are set, tighten down the nuts, locking the mags firmly in
place. The recommended torque for the nuts is 17
ft/lbs maximum. If you go for gorilla torque it, the mounting flange may
be damaged (or it may even break) and the mag must
be removed and replaced.
Then check your work one more time. If all's well, remove the timing
box. Close up the timing plug and safety-wire it, if you
are working on a so equipped Continental. Put a little anti-seize on the
threads and reinstall the No. 1 top spark plug you
removed earlier. Torque it to 30 to 35 ft/lbs and attach the ignition
lead.
Hook up the P-leads, other wires and ignition harness caps, in that
order. Be sure not to over-torque the P-lead nut. To quote
a Slick service tip, "it is extremely important to observe the 13 to 15
in/lb torque limit when tightening down the P-lead nut
onto the condenser stud." Exceeding the limit will break the condenser,
making it unsafe and requiring replacement (which
means going through all this again). Install the top cowl and lock it in
place with about half the fasteners.
Wrapping It Up
Remove all tools and such out of the way and recowl the engine. Fire it
up and do the standard run-up checks. Are the mag
drops within limits? There should be one, no matter how slight. Be sure
to turn the mags off for a second to be sure they do
indeed switch off and kill the engine.
Then shut down the engine and remove the top cowl. Check for oil leaks
at the mags and, if applicable, the timing plug. If
all's well, install the top cowl. Then do the necessary log entries.
Note that this model and serial number mag was removed
and this model and serial number mag installed in this position on the
engine, if a mag was replaced.
Don't forget the yellow tags. Many mechanics just staple them right to
the last page of the engine log book. If you don't put
them there, they must be kept in some other secure place.
Homepage URL: http://www.eaa49.av.org
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